Rubbers, such as packings for automotive, marine, and machine parts, and molded rubber products, such as tires and shoe soles, are indispensable to our daily life. Molded rubber products are currently used in many applications where elasticity is required but have the disadvantage of poor processability. Conventional methods for manufacturing molded rubber products include a series of time-consuming processes: rubber mastication→chemical mixing→cooling→heat input→sheeting→aging→cutting→press molding→trimming. Such methods have the disadvantage that the raw material tends to be lost during the processes. For example, the occurrence of flash during the press process is responsible for the loss of 5-10% of the raw material.
Press molding limits the design of molded rubber products because parting lines of the mold should remain only on the top and bottom of the products and the release of the products is possible only when the opening and closing sides of the mold are equal to or larger than the other portions of the mold, so that the mold can be demolded.
In an attempt to overcome this limitation, injection molding has long been applied in the rubber industry. However, injection molding still has problems in terms of processing despite no need for rubber sheet cutting, press molding, and trimming. That is, injection molding includes upstream processes before cutting, i.e. rubber mastication→chemical mixing→cooling→heat input→sheeting. Thereafter, the resulting rubber sheet is produced in the form of strips, which are fed into a hopper of an injection molding machine. The reason for the additional process for the production of the rubber sheet in the form of strips is that the rubber compound has low hardness and high surface tackiness, and as a result, its pellets are very difficult to feed into the hopper, unlike plastics. A process for dusting the surface of the strips is further needed to remove the tackiness of the strips, making the procedure cumbersome. Further, dust is produced in the workplace during the dusting process, resulting in an extremely poor working environment. Moreover, many voids between the strips make the transport of the strips troublesome and uneconomical.
In the case where the rubber compound is in the form of pellets instead of strips, the raw material is free to move, enabling the development of efficient machines, such as multi-station injection molding machines. Thus, pellets of rubber compounds for injection molding have been investigated in various fields due to their advantages. For example, Korean Patent Publication No. 10-2004-0050165 (entitled “Pellet-type rubber composition for shoe outsole, method for preparing the rubber composition and method for manufacturing shoe outsole”) proposes a composition including a base material, a filler, a coupling agent, a crosslinking agent, and a co-crosslinking agent wherein the base material is composed of 60 to 90 parts by weight of a rubber and 10 to 40 parts by weight of a thermoplastic resin. This composition is excellent in mechanical strength and various physical properties but has extremely poor flowability, requiring very high injection pressure, extremely high moldability, and the use of an expensive injection molding machine.
These requirements contradict the primary purpose of pursuing efficiency of the equipment. Further, the flow of the composition does not reach the fine portions of the mold, making it difficult to manufacture molded products with attractive appearance. This difficulty can be solved by mixing a softener, such as process oil, with the composition. In this case, however, the outer surface of the pellets becomes tacky, causing aggregation of the pellets. This also contradicts the purpose of the invention.